2006 Mustang GT completely stocked. What parts to replace?

I have a 2006 Mustang GT automatic completely stocked. It has 120,000 miles on it and I believe this is the time to brace myself to change major parts like the engine, transmission and so on.I love the body style on my car and I'm not impressed with the new body styles of the 2010 and up (my opinion). Instead of buying a new mustang GT, what can I change in my car for better performance? What should I start with first? Or wait until the parts need to be changed? I'm looking to spend between $9,000 - $20,000 for upgrades.

RIght, unless you do want to replace the engine with something bigger.
Start with a new rear gear ratio: 3:73:1 would be good for your automatic.
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Listen to Tom ...his advice is good. But on the off-chance you live in Calif, you can't do the cats, it's "tampering" with emissions and will not be able to register it. Otherwise, happy, safe driving! Way cool car.

This is moot and not of use to you Danny if you are not in Calif but Whoops, I was wrong. Looks like you can as long as it meets the emissions standards at tail-pipe. For CO2, (carbon dioxide), and 02 [no max given] and CO (carbon monoxide) Max is 1.00% @ idle-- and HC (hydrocarbons) Max. is 100 ppm@ idle and 130 ppm @ 2500 rpm http://www.arb.ca.gov/msprog/aftermkt/aftermkt.htm

Tom,
I would like to eventually change the engine for something bigger. The new mustang GTs have 400hp 5.0. I don't want anything way crazy, but to meet 400hp or a little stronger. If I replace for a new engine, what ripple effect would that cause? I'm sure I'd need new additional parts.
By the ways, thanks for the responses guys, I knew this place would be helpful. I used to live in Arizona (where family is now). I'm currently stationed in Virginia.
- Danny

It IS a thing of beauty. But sorry, never built one myself, so really can't answer your question. Accurately anyway. IMO the 5.0 is one of the best engines Ford,-- or anybody else for that matter-- ever built street-legal This it it

Coyote motors are sweet! Might need other parts and modifications, but it
can be done with the right knowledge and money. If you have 10-20K in
money to mod out the car, why not trade yours in for a newer one, 2012?

My brother has a 2002 Mustang GT. It has the bullet wheels on it. I'm trying to talk him into 17x10.5 wheels for the rear. Would he have to buy a whole set?, or can he just buy the rears?, will they m...